Fast optical signal (EROS)

The fast optical signal, measured as the event-related optical signal (EROS), attempts to detect neuronal activity optically on a millisecond timescale rather than through the seconds-slow haemodynamic response. Its proposed basis is that active neural tissue changes its light-scattering properties (through cell swelling and membrane-potential-linked refractive-index changes), altering the amount of near-infrared light reaching a detector within milliseconds of firing.

EROS is scientifically appealing because it would combine EEG-like timing with fMRI-like localisation non-invasively. In practice its amplitude is extremely small relative to noise, requiring extensive averaging, and independent replication has been limited; it remains a research technique whose signal-to-noise and even its physiological interpretation are actively debated rather than an established BCI input.